1. Field of the Invention
The present invention relates to a device for use in a chain saw and for adjusting tension of a saw chain (chain) which is wound around and rotates around a guide bar.
2. Description of Related Art
In a chain saw, tension of a chain is adjusted by moving a guide bar in a longitudinal direction with respect to a sprocket which drives the chain, to adjust a distance between the sprocket and the guide bar.
As this type of chain tension adjustment device, there may be a configuration in which a tension operating device is incorporated into a chain cover outside a guide bar, and a configuration in which a tension device is incorporated into a chain-saw main body on which a guide bar is mounted.
In the former configuration, there may be disadvantages that it is required that an operating portion provided on the chain cover and a moved portion in the guide bar be engaged with high accuracy, and the assembling process may become difficult. In the latter configuration, such disadvantages can be avoided.
The latter configuration in which the entire tension device is incorporated into the chain-saw main body is disclosed in JP H04-72681 B, and is as follows. A helical gear fixed to an operation shaft is engaged with a helical gear fixed to an end of a screw shaft member. When the operation shaft is rotated around the axis by a driver, the screw shaft member is rotated around the axis thereof. Thus, the guide bar is moved in the axial direction together with a member which is screwed to the screw of the screw shaft member and is engaged with the guide bar, so that the tension of the chain is adjusted.
In the configuration disclosed in JP H04-72681 B, a gear mechanism for rotating the screw shaft member is constituted by a single-stage gear unit (one drive gear and one driven gear are provided). In this configuration, since it is required that the operation shaft and the screw shaft be spaced apart from each other by a predetermined distance, for example, the sizes of the gears thereof are increased to enable the engagement therebetween. In particular, since the size of the driven gear, the radial direction of which is perpendicular to the surface of the guide bar, is limited by an installation space, the size of the drive (operating) gear should be relatively increased.
Accordingly, a gear ratio (the number of teeth of drive gear/the number of teeth of driven gear) is increased, a pitch of the gear is also increased, and thus, a backlash is also increased. As a result, it may be difficult to perform smooth and fine adjustment of the tension of the chain, and an operating force needed for the adjustment may also be increased.
Moreover, due to the large gear, the weight of the entire gear mechanism may be increased.
In addition, since the operation shaft and the axial direction of the screw shaft are perpendicular to and offset to each other, helical gears are used in the single-stage gear unit. Since a cutting process is necessary to form the helical gear, the cost may be increased.